Field of the Invention
The invention, some embodiments of which are described in the present specification, relates to a magnetic thin film comprising an ordered alloy. More specifically, some constitutional examples of the embodiments relate to the magnetic thin film in which the ordered alloy comprises Fe and Pt as main ingredients and further comprises Sc. Further, the invention, some of embodiments of which is described in the present specification, relates to an application device comprising the above-described magnetic thin film.
Description of the Related Art
The application device comprising the magnetic thin film includes a magnetic recording medium, a tunnel magneto-resistive element (TMR), a magneto-resistive random access memory (MRAM), a micro electromechanical system (MEMS) device, and the like.
The magnetic recording medium will be explained, as the first example of the application devices comprising the magnetic thin film. The magnetic recording medium is used in a hard disc drive, a magneto-optical (MO) disc, a magnetic tape, and the like. Magnetic recording systems used therein include an in-plane magnetic recording system and a perpendicular magnetic recording system.
The in-plane magnetic recording system is a conventionally used system wherein magnetic recording is conducted horizontally with respect to the surface of the hard disc, for example. However, the perpendicular magnetic recording system have been mainly used in recent years, in which magnetic recording is conducted perpendicularly to the surface of the disc, and higher recording density can be achieved.
A magnetic recording medium using in the perpendicular magnetic recording system (hereinafter, also referred to as “perpendicular magnetic recording medium”) at least comprises a non-magnetic substrate, and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; a seed layer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
An urgent need for reduction in the grain diameter of the magnetic crystal grains in the magnetic layer arises in recent years, in order to further increase the recording density of the perpendicular magnetic recording medium. On the other hand, the reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization. Thus, the magnetic crystal grains in the magnetic layer need to be formed of materials with higher magnetocrystalline anisotropies, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains.
However, the magnetic recording medium having the magnetic recording layer formed of the material with higher magnetocrystalline anisotropies possesses a high coercive force which makes it difficult to record magnetization thereon. In order to overcome this recording difficulty, energy-assisted magnetic recording systems such as a heat-assisted magnetic recording system and a microwave-assisted magnetic recording systems have been proposed. The heat-assisted magnetic recording system utilizes temperature dependence of a magnetic anisotropy constant (Ku) of the magnetic material, that is, a property that the Ku decreases as the temperature rises. A head having a function to heat the magnetic recording layer is used in this system. That is, writing is conducted during a magnetic switching field is reduced by raising the temperature of the magnetic recording layer to temporarily decrease the Ku. Once the temperature drops, recorded magnetization can be held stably, since the Ku returns to the original high value.
The tunnel magneto-resistive element (TMR) and the magneto-resistive random access memory (MRAM) involving the TMR will be explained, as the second example of the application devices comprising the magnetic thin film. Conventional memories such as a flash memory, a static random access memory (SRAM), and a dynamic random access memory (DRAM) record information with electrons in a memory cell. On the other hand, the MRAM is a memory using a magnetic body, which is the same as the hard disc and the like, as a recording medium.
The MRAM has an address access time of approximately 10 ns and a cycle time of approximately 20 ns. Therefore, the reading/writing rate of the MRAM is approximately five times faster than that of the DRAM, and comparable with that of the SRAM. Further, the MRAM has advantages of low power consumption that is approximately tenth of that of the flash memory, and capability of higher density integration.
The TMR used in the MRAM can be produced by various techniques. For example, a stacked body comprising the TMR can be obtained by forming a ferromagnetic thin film onto an anti-ferromagnetic thin film. Japanese Patent Laid-Open No. 2005-333106 discloses an exchange-coupled device in which an anti-ferromagnetic layer and a ferromagnetic layer which is exchange-coupled with the anti-ferromagnetic layer are sequentially stacked onto a substrate, and the anti-ferromagnetic layer comprises an ordered phase of an Mn—Ir alloy (Mn3Ir). A schematic cross-sectional view of the TMR including the above-describe exchange-coupled device is shown in FIG. 5 of the above document. Further, a spin-valve magneto-resistive element comprising the exchange-coupled device is shown in FIG. 4 of the above document.
The micro electro mechanical system (MEMS) device will be explained, as the third example of the application devices comprising the magnetic thin film. The MEMS device is a general term for devices in which mechanical component parts, sensors, actuators and/or electronic circuits are integrated onto a single substrate. The useful substrate includes a silicon substrate, a glass substrate, and a substrate of organic material. Exemplary application of the MEMS devices includes a digital micro mirror device (DMD) which is one of optical elements in a projector; a micro nozzle used in a head of an inkjet printer; and various sensors such as pressure sensors, accelerometers, and flow sensors. In recent years, it is expected that application of the MEMS device will be developed in a field of medical care, in addition to manufacturing industry.
There is a need of improvement in magnetic properties of the magnetic thin film, particularly increase in a uniaxial magnetic anisotropy constant (Ku), in all of the above-described application devices (the magnetic recording medium, TMR, MRAM, and MEMS device). In this context, it is considered that development of the magnetic thin film exhibiting such excellent Ku will greatly contribute to increase in capacity and/or density of the recording medium and memory.
Ordered alloys have been received attention and investigated actively, as a candidate for a material exhibiting the excellent Ku. Interested ordered alloys include various materials such as FePt and CoPt. Search of additive material which is added to these material to improve properties have been continued. In this context, Japanese Patent Laid-Open No. 2010-135610 proposes a magnetic thin film containing an L11 type Co—Pt—C ordered alloy. This ordered alloy optionally comprises at least one additional element selected from the group consisting of Ni, Fe, Mn, Cr, V, Ti, Sc, Cu, Zn, Pd, Rh, Ru, Mo, Nb, Zr, Ag, Ir, Au, Re, W, Ta, Hf, Al, Si, Ge, and B. However, influence of these additional elements on the magnetic thin film has not been investigated concretely.
Japanese Patent Laid-Open No. 2008-059733 proposes a magnetic recording medium having a magnetic recording layer with a granular structure, wherein the magnetic recording layer consists of magnetic crystal grains consisting of an L10 type ordered alloy such as FePd, FePt, CoPt and MnAl, and a non-magnetic crystal grain boundary consisting of an oxide. Here, the oxide consists of oxygen and one or more element in which at least one of the elements has a negative reduction potential. Scandium (Sc) is described as an exemplary element which constitutes the oxide. However, the above described magnetic recording layer with the granular structure is not evaluated in the above document. In addition, the above document does not teach or suggest introduction of an additional element to the L10 type ordered alloy.
As described above, it is the current state that Sc as the material to be added to the ordered alloy is little investigated. There is almost no advance in the investigation of magnetic properties of the ordered alloys to which Sc is added, especially the magnetic anisotropy constant Ku of such ordered alloys.